Single overload fatigue crack growth retardation : an implementation of plasticity induced closure




Kirmani, Ghulam Ashraf-Ul-Harmain

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Fatigue life prediction following overloads is required in such applications as, aerospace, automobile, and pressure vessels industries for damage tolerant design. Modelling of life prediction in fatigue is complicated by a host of variables which includes loading history. The characteristic features which result as a posteriori evidence of the loading history include overload plasticity zone, crack closure with a special trend with respect to crack length, spike-dip in fatigue crack growth rate and retardation in fatigue growth. This research focuses on life prediction of components subjected to variable magnitude single overloads, in a cyclic loading situation. This thesis introduces the plasticity range interaction, and closure effects for variable magnitude single overload problems. A simple model is presented which captures these characteristic features following overloads. A detailed study on the crack-tip plasticity is conducted to identify the dimensions of the plasticity zone. A new approach is presented which is useful in obtaining suppression factor for fatigue growth retardation. This factor is required in fatigue crack growth models to account for retardation effect following overloads. The model for fatigue crack growth is tested for constant amplitude loads. A detailed study is presented on fatigue crack closure based constant amplitude calculations. Two different approaches to fatigue growth calculations are presented. An assessment of the errors that occur in assumed-crack extension method is also presented. Several examples have shown a good agreement between experimental and theoretical results. The study is extended to variable magnitude single overload problem for determination of fatigue growth calculations. Two different approaches have been adopted, one based on plasticity range interaction, and the other on closure. It has been shown that the two approaches are equivalent. There is an excellent agreement between predictions and theory for fatigue life calculations and fatigue growth rate. This research directly contributes to life prediction under single overloads without reliance on data fitting. It has tremendous potential for fatigue life prediction under programmed block loads, multiple overloads and finally for random loads, which can be investigated in further studies.



Plasticity, Metals